Gallium nitride (GaN), aluminum nitride (AlN), and indium nitride (InN), which are nitride semiconductors, or a material, which is mixed crystal thereof, have a wide band gap, and are used for a high-power electronic device, a shortwave light-emitting device, etc. For example, GaN, which is a nitride semiconductor, has a band gap of 3.4 eV that is larger than a band gap of 1.1 eV of silicon (Si) and a band gap of 1.4 eV of gallium arsenide (GaAs).
The high-power electronic device may be a field effect transistor (FET), or more particularly, a high-electron-mobility transistor (HEMT). The HEMT using the nitride semiconductor is used for a high-power high-efficiency amplifier, a high-power switching device, etc. More specifically, in an HEMT, in which aluminum gallium nitride (AlGaN) is used for an electron supply layer and GaN is used for a transit layer, a difference in lattice constant between AlGaN and GaN causes a distortion, resulting in that piezoelectric polarization and spontaneous polarization occur in AlGaN, and high-density two-dimensional electron gas (2DEG) is generated. Hence, an operation with a high voltage is available. The HEMT may be used for a high-efficiency switching element, and a high breakdown voltage power device for an electric vehicle etc.
Japanese Laid-open Patent Publication No. 2002-359256 and No. 2011-030396 are examples of related art.
A field effect transistor using silicon as a semiconductor material spontaneously contains a body diode. The body diode is connected to the transistor by anti-parallel connection. Hence, even if a high surge voltage is generated, an avalanche decay occurs. Thus a sufficient surge resistance is provided. However, a GaN-base HEMT does not spontaneously contain the body diode. If a high surge voltage is generated, the HEMT may be broken and a failure or the like may occur. Hence, a surge protector, such as a varistor or a resistance-capacitance (RC) surge absorption circuit, has to be additionally provided.
The surge protector typically has a large parasitic capacity. When the HEMT etc. is operated, heat is generated, and the temperature is increased. The increase in temperature decreases the operation efficiency and operation speed. Owing to this, a switching loss is generated when the surge protector is used for the switching element. Also, even when the HEMT is normally operated, flow-through current likely flows in the surge protector, and power consumption tends to be increased. Further, the operation speed of the HEMT is higher than the operation speed of the surge protector.
Even when the surge protector is provided, current flows in the HEMT before current flows in the surge protector. The HEMT may be broken.